This invention relates to hydraulic systems, such as power, cooling, cutting, etc., including a working-liquid pressure line and drain line communicated accordingly with a means for maintaining the working properties of the liquid.
The latter means in known such systems (Dudley A. Pease. Basic Fluid Power, Prentice-Hall, Inc., Eglewood Cliffs, N.J., 1967; Franklin D. Yeaple. Hydraulic and Pneumatic Power and Control, McGraw-Hill Book Co., New York, 1966; U.S. Pat. No. 3,945,208, etc.) removes solid contaminants from working liquid by filtering, straining, gravitational displacement, centrifugal separation, etc. with full flow and bypass (5-20% of the flow).
It is well known that reliability and longevity of hydraulic systems, in many respects, depend on the working properties of the used liquids.
These properties are determined by the presence of the solid, gaseous and liquid contaminants in the liquid, their fineness and the state of their dispersion.
The solid contaminants are the products of wear (metal filings, rubber, etc.) and oxidation of both the hydraulic units and the working liquid itself, or are the dust (the most destroying contaminant) from the atmosphere.
The intensity of the contamination from the atmosphere in hydraulic systems (especially those of farm, road-making, construction and the like machinery) depends on liquid volume oscillations in their tanks when operating. The oscillations are basically dictated by the work of their hydraulic cylinders and accumulators.
The matter is not only that the solid contaminants are abrasive, cause wear and decrease the term of hydraulic unit service many times, but they may wedge movable tailings (especially the plunger ones) and be the cause of the inoperativeness of automatic controls.
The gaseous contaminants (mainly air and water vapor) are absorbed from the atmosphere (as above) and deteriorate the pliability of hydraulic systems, their triggering, stability, causing also inoperativeness.
Oxygen solubility in liquid being higher than that of atmosphere air, the dissolved air contains 40-50% more oxygen, and the oxidation of hydraulic units and liquid is intensified.
Also, gas forms foam oxidizing still more because of bigger interface and more oxygen content. In time, the stable foam forms viscous contaminants depositing on the surfaces of hydraulic units. Foam-forming is sharply increased by water presence, even at only 0.1% water.
Gas is always present in liquid, at least in a dissolved form, and usually does not affect liquid mechanical properties. But vibration, decreasing pressure and heating give the gas off (even with foaming) and form an inoperative gas-liquid mixture instead of the former solution fitted for work. That is why the problem especially arises in hydraulic systems of vehicles when the systems remaining inoperative are subjected to jarring and vibration. This may aggravate starting such a hydraulic system.
Water contaminants come into "breathing" tanks (as described above) in vapor form and condense when temperature drops, with the deteriorating effects described above.
Mentioned known filtrating means remove only solid contaminants with the size of the solids being more than the clearance in sliding pairs of hydraulic units. Here the clearance is usually expressed in terms of the difference between the diameter of a hole and the diameter of the mating part which assembles into it, e.g. the difference between the diameters of a cylinder and plunger. In many cases this is achieved by fine-mesh bypass filters consuming energy and requiring frequent change because of their clogging and, in some areas, becoming a repository for biological growth.